This invention relates to automatic quench compensation for liquid scintillation counting systems, and more particularly to a method of restoring at least one endpoint of a quenched sample spectrum of counts-per-minute versus energy-of-events substantially up to a previously established counting window discriminator level for an unquenched sample spectrum using a signal varying as a function of quench.
Prior art radiation accumulation instruments suffered from system instability due to variations in photomultiplier gain, amplifier gain, counting window or discriminator level stability and similar effects which were caused, for example, by variations in temperature, line voltage or other environmental or instrument related effects. One early application for such instrumentation, for instance, was in the well-logging area such as disclosed in Silverman, Youmans et al. and Scherbotskoy, cited above. When dropping a probe containing a photomultiplier tube down in a deep well, extreme changes in temperature were encountered, resulting in variations in photomultiplier gain and amplifier drift for example, among a number of other effects as mentioned in columns 1 and 2 of Youmans et al. The Silverman patent attacked this problem by correcting for these drifts by transmitting a set of calibrating signals from the same location in the well, proportional to a substantially constant quantity, and comparing the amplitude of these signals with the amplitude of the sample signals. Youmans et al. improved upon this approach by varying the operating characteristics of at least one element in the scintillation counter system such as by changing the gain of an amplifier or photomultiplier tube. Scherbotskoy transmits such a standard and uses it to adjust the counting channel discriminator windows to shift them with regard to the position of the spectrum.
As stated above, all these systems were concerned with stabilizing the behavior of the instrument itself. Due to improvements in instrument components, and since present day soft beta liquid scintillation counting systems are not normally operated under extremes of environmental conditions, this type of effect is not a significant factor in degrading the operation of such equipment. The equipment itself has more than adequate stability and reproducibility for the application. If the user would restrict himself to very small samples of a chemically innocuous nature, such as labeled toluene, then all the samples would have the same spectrum endpoints and the same counting efficiencies in the various discriminator ranges or counting windows used. However, users frequently desire to put samples in these equipments which are chemically active and which reduce the light output of the transducing liquid scintillation solution in which they are dissolved. This reduction in light output, due to the chemical interference of the sample, is called "quenching"-- this case, "chemical quenching." It is also possible, of course, to have color quenching where the optical characteristics of the solution containing the sample vary. Examples of sample materials which cause chemical quenching are fatty acids, phosphates and halogenated hydrocarbons. Sometimes samples are obtained dissolved in a quenching solvent, such as a lipid sample dissolved in chloroform. The chloroform containing the sample is then added to a liquid scintillation mixture. The chloroform will quench the light output of the liquid scintillator, shifting the spectrum endpoints to lower values. The result of the spectrum shift is that the spectrum endpoints are no longer aligned with the previously established counting window discriminator levels and counting efficiencies in the windows are changed as a result of the sample quenching.
As disclosed in the Packard patent cited above, the prior art practice for quench calibration involves the preparation of a set of calibration curves, determined in advance for each different sample volume that may be encountered. This is done by preparing a series of samples of known activity for each different isotope that may be of interest. A different amount of quench material is then added to each sample in each series. The differently quenched samples in each series are then counted in an environment free of any standard so as to determine counting efficiency for each varying degree of quench, and in an environment exposed to a standard, such as the external-standard in the Packard case, or by using the external-standard channels-ratio method as disclosed in Nather cited above, so as to determine the counting efficiency for the standard for each varying degree of quenching. Based upon these data, a set of calibration curves are prepared for each isotope of interest and for each sample volume. Thus, the two observed counts for each unknown test sample are compared to determine counting efficiency at true sample activity levels.
In order to maximize the use of this approach, Packard, referenced above, discloses what is called "balance point operation" where the peak of the spectrum of counts-per-minute vs. pulse height or energy is centered in the discriminator windows so that small variations to the left or right of the peak result in relatively small changes in counting efficiency within a particular window. However, when increased quenching is encountered, this system, which only minimizes the error at best, falls down and the spill-over from one discriminator window to another so degrades the resulting data that they may become completely ineffective. A "worst" example is spill-over from one discriminator window into another when performing dual label sample counting.
Accordingly, the problem is to make measurements under conditions where the molecules of the sample are intimately mixed with the molecules of the transducer, as in liquid scintillation, and where there is reduction of light output from the transducing liquid scintillation solution due to chemical or color interference of the sample which is in intimate contact with the transducer.
In any liquid scintillation spectrometer, the basic principle of nuclear detection is the same. A radioactive isotope decays and emits a number of subatomic particles. The beta particle, which is of primary concern in many applications, travels only a short distance and is extremely difficult to detect as a first order effect. Accordingly, a transducer is employed whose chemical combinations result in molecules excited into the singlet state by the near passage or collision with the beta particle. When the molecules return to their ground state, photons of light are radiated in all directions, generated as a result of the beta particle and of a much lower wavelength. The light, accordingly, penetrates the body of the chemical solution in which the isotope is dissolved or suspended and passes out of the container.
Beta particles are emitted from a given nuclide with a distribution of energies. When they are detected via the liquid scintillation process, the energy is released from the vial as "light," which in turn is converted into an electronic pulse. A number of photons is produced, proportional to the energy of the initiating beta particle, and the size of the electronic pulse is proportional to the number of photons which produced it and the efficiency of energy transfer in the vial. When quenching occurs, the light or photon output is reduced.
Anything which impedes the passage of the photons out of the container is known as a quenching agent. There are two general classifications as mentioned above-- color and chemical quenching. Color quenching decreases the number of the photons leaving the container by absorbtion. Chemical quenching destroys the chemical ability of the transducer to produce photons. Both types have the apparent effect of shifting the apparent beta energy spectrum, counts-per-minute vs. energy, toward zero. The effect of quenching has to be taken into consideration to prevent major errors in the resulting data, making it of little value.
This has been known for some time and, as recited above with regard to Packard, graphs have been prepared using samples with known amounts of radioactivity and quenching agents to provide a means by which accumulated data can be corrected within limits of statistical error. However, as the quenching increases, the error involved increases at an exponential rate until it reaches such a magnitude that the data accumulated again becomes meaningless. In the prior art, samples having a wide range of quenching are grouped and the instrument is recalibrated to function with the more highly quenched samples, for instance, after the less quenched samples have been processed.